Vitamin D is well known for its essential role in monitoring phospho-calcium homeostasis and in bone mineralization. The synthesis of vitamin D is initiated at the level of the skin under the action of UVB rays and their biologically active form; calcitriol is then produced in the kidney to exert its functions.
Nevertheless, recently, it is known that there is also an autonomous cutaneous system for the production of calcitriol at the level of the skin, and the discovery of this system made it possible to reveal the involvement of vitamin D in other cutaneous cellular functions: monitoring of the cellular growth and differentiation, cytoprotective and immunomodulatory role.
The local production of calcitriol at the level of the skin results from several successive stages initiated at the level of the epidermis that contains its precursor, the provitamin D. Under the action of the UVB rays, the provitamin D is converted into previtamin D3, itself transformed into vitamin D3 that is then activated at the level of keratinocytes in calcitriol. The thus activated vitamin D is capable of exerting autocrine and paracrine effects by means of a receptor, the vitamin D receptor (VDR). By this method, vitamin D regulates numerous processes involved in cutaneous homeostasis: the formation and maintenance of the epidermal barrier, capillary growth, the innate immune system, and ageing.
The synthesis of vitamin D is under the influence of numerous factors: internal factors such as age or phototype, external factors such as the season, air pollution or the intensity of UV radiation, or else behavioral factors such as wearing clothing, the use of sunscreen, etc. In addition, it is known that insufficient vitamin D is very common and rising steadily, in particular in older individuals and women who are the populations the most at risk.